Ever since the first commercially-viable gaze tracking solution [1] in 1974, the challenge for all subsequent systems has been to provide high gaze tracking accuracy with low hardware complexity. Present day commercial systems trade-off low hardware complexity in favor of high gaze tracking accuracy and consequently tend to be complex, expensive and restrictive in terms of hardware integration.
In order to ensure gaze tracking accuracy and system robustness, the solution disclosed in [2] uses both on-axis (or bright pupil response) and off-axis (or dark pupil response) infrared (IR) illumination (active IR). This translates to increased hardware complexity and power consumption, as well as system miniaturization limitations.
A similar approach, disclosed in [3] by Tobii Technology Ab, includes a single camera system, using both on-axis and off-axis IR illumination. Another solution disclosed by Tobii Technology Ab [4], relies on the same on-axis-off-axis IR illumination setup and a dual-camera system. Another gaze tracking system along the same lines is the one disclosed by Seeing Machines Pty Ltd [5], including a stereo-camera setup embodiment, the second camera being optional. Utechzone Co. Ltd disclosed in [6] and [7] two single-camera, off-axis IR illumination solutions that follow the same, aforementioned, trade-off. Similar methods were also disclosed over the years in the scientific literature, e.g., [8], [9], [10], all relying on the active IR, on/off-axis illumination paradigm.
The accuracy and robustness requirements for a commercially-viable system and method have steered the current gaze tracking paradigm towards the following characteristics:                active IR, to simplify the image acquisition process and allow for a glint-based Point-of-Gaze (PoG) computation;        a PoG computation on a graphical display, requiring an initial calibration step and a Pupil Center Corneal Reflection (PCCR) vector to map the PoG from the image coordinate system to a reference coordinate system;        on-axis and off-axis illumination for pupil extraction robustness and dual glint availability for depth and perspective correction; and        below the graphical display surface positioning to maximize the availability of the glint for detection and extraction.        
All of these solutions are done in order to guarantee an angular accuracy of the estimated PoG within the current industry standards regardless of external constraints such as frame rate, user variability, illumination conditions, effective range, etc.
If the main requirement is low-complexity hardware and system specifications, the current trade-off is the system's gaze tracking accuracy and robustness, e.g., [11]. The gain in this case is hardware simplicity, device size and spatial positioning. The reported gaze tracking angular accuracy of the system disclosed in [11] has been found to be between three to six times worse than the current industry standard.
The trade-off between hardware complexity and gaze tracking accuracy has so far made commercially available gaze tracking systems impractical for physical integration within computing devices. Due to the positioning restrictions, i.e., at the bottom of the graphical display, the use of these systems is either as stand-alone, external devices or as impractical integrated solutions, e.g., [12]. Their integration typically requires a major redesign of the computing device, process which is both non-trivial and production-cost-ineffective.